Carb Sync issues

Hey Gang,
While I am flushing and cleaning out my rusty tank I thought I would give the carbs a sync. I hooked up the tubes to the ports in the side of the block and started it up. At first I was really happy since all the needles were in the same position. This will be easy! Then I noticed all the gauges pointed at zero with a couple bouncing a bit. The bike starts up and runs fine so how could I be pulling zero vacuum? I have not looked at the valves yet (it's on my list) but I did do a compression check and it was low across the board. What am I missing?

 

If the bike runs fine and pulls strong, I would bet the problem has to do with the gauges. My synchronizer has multiple jets for the vacuum lines and will give false readings if the wrong ones are used. Also, when I checked my compression with a Harbour Freight gauge it was way low. but checked fine when I used a quality gauge. When you say it runs fine I'm assuming you mean rides fine with good power, otherwise it may be something other than the above. Good luck!

 

It may not directly answer your question but this video just may be of interest if you are messing with carbs...

https://www.youtube.com/watch?v=c9V1GvczIFE&list=UUOXDrrwQRZqOQ3iUCXN12rw

Hope it helps


SkiMad

 

Dr. Jay, I used an older Craftsman pressure gauge and I got 90-105 but the engine was cold. I have not tried a leak down test but I probably should. I also need to get after the valves. It runs pretty good.

Great video SkiMad. There's probably a lot of truth to what he says. I wanted to sync my carbs because I have replaced a couple of the bodies and am fighting a lean surge. I'm definitely going to do the plug check.

 

Another thing, when you checked the compression did you not only hold the throttle wide open, but also use something to prop the constant velocity slide wide open? If not, that plus the cold engine could be the culprits.

 

Dr. Jay, When I checked the compression the carbs were off the bike. I am planning to do a leakdown test this weekend if I have time.
Also, I replaced the hoses on the sync gauges because they were a little loose and got slightly better readings but they are all less than 5 hg when on the gauge it says 20 is normal. Someone told me to ignore the amount of vacuum and worry them all being the same. Is that right?

 

Couple of things to remember when doing synch: If the bike starts and idles fairly well, you'll never need to turn a synch screw more than 1/4 turn, probably less; and blip the throttle a few times after any screw adjustment to allow the linkages and throttle plates to settle into their new position.

 

I've been away for a few days but am wondering if you did the leak down test and what the results were. I'm puzzled as to why you are observing the things you are if the bike really runs well. Does it have good torque and pull strongly up to 9000 rpm or just go down the road with acceleration no better than the average car? If the gauges are accurate, I don't think it should run very well because the symptoms are suggestive of bad rings or valves although tight valve lash could also be the culprit.